A number of interesting electro-optic display media have potential application in displays possessing good optical appearance, large area potential, flexible substrates, low cost, and ease of manufacture. Such display media include microencapsulated electrophoretic displays, rotating bichromal ball displays, suspended particle displays, and composites of liquid crystals with polymers, including polymer dispersed liquid crystals, polymer stabilized liquid crystals, and liquid crystal gels.
Such display media also include electrochromic media. An example electrochromic medium takes the form of a nanochromic film including an electrode formed at least in part from a semiconducting metal oxide and dye molecules capable of reversible color change attached to the electrode. See, for example, B. O'Regan, et al., Nature, 353, page 737 (1991). Nanochromic films of this type are also described, for example, in International Applications Publication Nos. WO 98/35267 and WO 01/27690.
Practical and cost effective image element addressing techniques, however, remain elusive for such displays. For example, direct drive addressing provides control of each element, or pixel, through an external drive circuit. This scheme is both expensive and impractical for displays containing a large number of pixels or closely spaced pixels.
Active matrix addressing requires use of electrically non-linear elements on the display substrate. Non-linear elements include, e.g., transistors, diodes, and varistors. While this type of addressing is well-known and widely practiced, it is expensive to produce and difficult to achieve on plastic substrates.
A third means of addressing uses multiplexing, in which the conductive portions of the substrate are patterned so that rows of pixels on the substrate are electrically connected to each other, and columns of pixels on the substrate are also electrically connected to each other. Typically, voltages are sequentially placed on the row electrodes, with the pixel data for each row corresponding to voltages applied on the column electrodes.
Multiplexing, and other passive addressing schemes, are used in combination with a variety of display media. Use of passive addressing is limited, however, to displays in which the optical response as a function of applied voltage is non-linear and/or in which there is a significant voltage threshold to turn on the pixels. Display media which do not show a pronounced voltage threshold show poor contrast when driven with multiplex addressing schemes.
More specifically, standard passive addressing schemes work poorly with many particle based, electrophoretic displays, and some other types of electro-optic displays. Addressing such electro-optic media-based image elements by combined application of voltages on neighboring row and column electrodes generally requires that the medium exhibit a threshold voltage response effect. Alternatively, the medium may exhibit a steep, nonlinear response to the applied voltages. Barring this, application of a voltage on one row of electrodes (or column) typically causes an image response along the entire row, defeating the ability to address, and cause an image response in, a single element or pixel.
Some image forming techniques employ a photoconductive layer rather than matrix addressing techniques. Typically, a voltage is applied across two plate electrodes that reside on either side of an electro-optic display medium. Rather than using row and column electrodes, the plate electrodes cover the entire display area, and a photoconductive layer lies adjacent to the display medium between the plate electrodes. Illumination of the photoconductive layer with a light image causes imagewise changes in conductivity of the photoconductive layer, corresponding to the imagewise distribution of light striking the photoconductive layer. This in turn causes imagewise increases in the voltage drop appearing across the display medium.
Hence, the display medium produces a visible image that corresponds substantially to the imagewise variations of the illuminating electromagnetic radiation. One limitation of such an approach is the need to create a light image to produce a corresponding image by the display. Further, the size and orientation of the image displayed depends on the size and orientation of the image contained within the illuminating radiation.